This invention relates to a procedure and device for the specific manipulation and/or deposition of microscopic particles in high-frequency plasma.
As is generally known, formation of high-frequency plasma in the respective reaction gas is a suitable means for achieving the desired degradation reactions or the like for processing or degradation procedures such as plasma etching, or chemical vapor deposition (CVD). To optimize CVD applications, e.g., for separating amorphous, hydrogenated silicon (a-Si:H) for photovoltaic devices, thin-film transistors, flat-screen displays or color detectors in imaging systems, there are numerous studies on how the properties of deposited layers depend on plasma parameters, e.g., the types of reaction gas, HF voltage or gas pressure. It has been shown that microscopic particles (so-called xe2x80x9cparticlesxe2x80x9d) can form in the plasma and have a disruptive or facilitative effect on the layer properties, depending on the application.
For example, in xe2x80x9cAppl. Phys. Lett.xe2x80x9d, Vol. 69, 1996, pp. 1705 forward, D. M. Tanenbaum et al. describe the formation of particles in plasma during a-Si:H deposition as follows: Negative ions are formed in the silane reaction gas as the result of electron bombardment, and react in the plasma with radicals and cations. This produces growing particles, which have a negative charge, as the electron velocities are significantly higher in comparison to the cation velocities. Due to the formation of space charge regions near the electrodes, these particles, which can grow to xcexcm dimension sizes, to not get to the substrate, which generally is secured to one of the electrodes. D. M. Tanenbaum et al. showed that, despite the space charge zone, particles ranging from roughly 2 to 14 nm in size reach the substrate during plasma discharge and, once there, can trigger disruptions in the layer properties.
In the xe2x80x9c14th European Photovoltaic Solar Energy Conferencexe2x80x9d (Barcelona 1997), Paper No. P5A.20, P. Roca i Cabarrocas et al. describe a significant improvement in charge carrier transport in a-Si:H layers by embedding particles. The particles arise under specific pressure conditions in the reaction gas, and are identified by characteristic, so-called xe2x80x9chydrogen evolutionxe2x80x9d measurements in the layer. The layers containing the particles exhibit a considerable increase in dark conductance and photoconductivity in comparison to amorphous layers. In addition, a considerable improvement was achieved in the stability of photoelectric properties under illumination.
One general problem in the previous studies on the effects of particles in CVD deposited layers is that a means for the targeted and reproducible handling of particles occurring irregularly in the reaction gas has thus far not been available. A particular problem in this case is that the particles can arise within roughly 1 second at the usual plasma frequencies of about 14 MHz.
Additional aspects of particle formation are illustrated below making reference to a conventional device according to FIG. 13.
In a plasma state, e.g., generated by a glow or gas discharge, a gas encompasses particles of varying charge, e.g., positively or negatively charged ions, electrons and radicals, but also neutral atoms. If microscopic particles (up to several 10 xcexcm in size), e.g., dust particles, form or exist in the plasma, these take on an electrical charge. The charge can reach several hundred thousand electron charges depending on the particle size and plasma conditions (type of gas, plasma density, temperature, pressure, etc.).
In the known device shown in FIG. 13, two flat discharge electrodes 11 and 12 are arranged one atop the other in a reactor (vessel walls not shown) with a carrier gas. The lower circular or disk-shaped HF electrode 11 is actuated with an alternating voltage, while the upper, annular counter-electrode 12 is grounded, for example. The electrode distance measures roughly 2 cm. A control circuit 13 is set up to connect the HF generator 14 with the HF electrode 11 and actuate the grounding and separation circuit 15 of the counter-electrode 12. The high-frequency energy can be injected with a frequency of 13.56 MHz and a power of roughly 5 W, for example. The carrier gas is formed by inert gases or reactive gases at a pressure of approx. 0.01-2 mbar.
A state of equilibrium preferably sets in among the particles, in which the gravitational force G acting on the particles is balanced with an electrical field strength E, to which the particles are exposed as the result of a space charge near the HF electrode 11 as a function of their charge. Also known is the formation of plasma crystalline states of particle configurations, but this is limited to particles with characteristic dimensions exceeding 20 nm, since the respectively carried charge is so low for smaller particles that thermal fluctuations have a stronger influence on the particles than the Coulomb interactions required for the plasma crystals, so that a uniform structure cannot be formed. In addition, formation of plasma crystals was previously limited to particles introduced into the reaction space from outside, e.g. dust particles. Therefore, a targeted handling of nanocrystalline particles, in particular with characteristic sizes of a few to several 10 nm, could not be derived from the manipulation of particles arranged in a plasmacrystalline manner.
However, in view of the known influence of structural or photoelectric properties of deposited layers resulting from built-in nanocrystalline particles, there is a strong interest in being able to control particle incorporation, in particular with regard to the type, size, number and position of the particles.
The manipulation of particles in a plasmacrystalline state is known from PCT patent application WO 98/44766 being published after the priority date of the present patent application. In JP 04-103769, a Laser-CVD-procedure is described.
Thus, it would be advantageous to provide a method for the specific manipulation or separation (deposition) of particles in or from plasmas, in particular, for influencing the particles themselves or modifying a substrate surface or a layer, and a device for implementing the procedure.
When exposed to a sufficiently energetic irradiation, which triggers in particular a discharging or reversing the charge of the particles, or exerts a light pressure, particles that arise internally in the reaction space with an ignited plasma, or are provided to the reaction space from outside (externally) and initially have a negative charge, are moved to an altered target position from an initial position corresponding to the force equilibrium of the negatively charged particles. The particles can have sizes ranging from several nanometers to roughly 100 xcexcm. The energetic irradiation can encompass laser radiation to trigger a discharge, a UV laser or electron irradiation for reversing particle charge via secondary electron emission, or light irradiation to generate a light pressure. The target position of the particles can be a range with altered plasma conditions, or a substrate on which the particles are applied alone or simultaneously with layer formation via plasma deposition.
The nanoparticles exhibit a substantially non-uniform spatial distribution in the plasma. This means that the nanoparticles are randomly distributed relative to each other, at statistically distributed locations. To this end, the conditions in the reaction space, in particular the plasma conditions, e.g., the ratio of electrons to ions in the plasma, are adjusted depending on the particles in such a way that the particles possess such a high energy that substantially no ordered or plasma crystalline states are formed.
A special advantage to the invention is that the energy-rich irradiation of the particles initially distributed substantially non-uniformly in the plasma takes place in a location-selective manner, so that particles are exposed in the form of a masking of altered plasma conditions in predetermined, selected plasma areas, or applied to the substrate based on a deposition pattern.
The equilibrium in particular between the gravitational force and electrical forces on the particles in the initial position can also be influenced by a location-dependent change in a static or low-frequency alterable electrical field between the electrodes of a plasma reactor (exertion of external adjustment forces). In this way, the particles in the plasma can be arranged on surfaces curved in whatever way with any edges. Therefore, the particles in the plasma can be moved in a predetermined manner, wherein this movement is even reversible, so that the particle arrangement can be adjusted between various conformations.
Another aspect of the invention is that the location-selective deformation of a substantially non-uniform particle arrangement subjects it to different plasma conditions in various partial subdomains. This enables a location-selective plasma treatment of particle areas (e.g., coating or stripping), in particular in plasma between two essentially flat electrodes. Application to a substrate can follow such a location-selective particle treatment.
In addition, an aspect to the invention lies in the fact that the formation of a particle arrangement is not influenced by the presence of a substrate in a plasma reactor, in particular, between reactor electrodes for generating a glow or gas discharge. In particular, it is possible to perform the aforementioned conversion processes in direct proximity to a tabular, flat or bent substrate, and then to reduce the distance between the particles in the particle arrangement and substrate surface in such a way that at least a predetermined portion of the particles is applied to the substrate surface. The reduction in distance can either be achieved by influencing the field strengths that hold the particles in position, or by moving the substrate surface. As a result, the particles can be deposited on substrate surfaces in patterns configured as desired. Therefore, the invention provides a novel, location-selective, mask-free coating procedure with which modified surfaces are generated. The applied particles give the modified surfaces altered electronic, optical and/or mechanical properties. However, it is also possible to use the particles applied in a location-selective manner themselves to mask or condition the substrate surface before or during an ensuing additional coating step.
A device according to the invention for manipulating particles encompasses a reaction vessel, which contains means for generating a plasma and at least one substrate. The means for generating the plasma preferably consist of tabular, essentially parallel electrodes, between which the substrate can be moved. The electrodes in the reaction vessel can exhibit field-forming structures for the location-selective influencing of the particles. The reaction vessel can also contain means for location-selective particle discharging (e.g., UV lighting means with a masking device), means for exposing the particles to radiation pressure, monitoring means and control means.
One special aspect of the invention involves configuring the electrodes for the location-selective influencing of particles in the reaction vessel. According to the invention, an electrode arrangement (or adaptive electrode) is described that exhibits numerous electrode segments, which are actuated substantially simultaneously with a high-frequency voltage, and each individually with a segment-specific direct voltage or low-frequency voltage. The high-frequency voltage generates or maintains a plasma state in the reaction vessel, while the direct or low-frequency voltage generates a static or slowly variable field distribution (field E) in the reaction vessel, during exposure to which the particles become arranged or move in the reaction vessel.
Additional features of the adaptive electrode include the formation of a matrix arrangement comprised of miniaturized electrode segments, the shaping of the matrix arrangement as an essentially flat, laminated component, whose electrode side faces the reaction vessel, and whose backside carries control electronics, the pressure relief of the component, e.g., via the generation of a vacuum in the space which the back of the electrode arrangement faces, and the provision of a temperature control device for the control electronics.